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Capítulo 12
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1. Different enzymes methylate different residues in histone molecules. It is not the methyl group itself that brings about a particular response, but the effect it has on the binding properties of the histone tail, according to the tenets of the histone code

2. A diploid cell contains about 6.4 X 10 9 bps of DNA; one nucleosome occupies about 200 bps, requiring 6 X 10 7 molecules of each core histone present at two per nucleosome. To produce this number of proteins during a relatively short S phase, each of the histone genes are present as part of the moderately repeated fracton of the genome, as discussed on page 417.

3. Cells possess more than one NLS receptor that recognize different localization sequences, and that the mutation was occurring in a gene that encoded one of these NLS receptor species.

4. 2 Barr bodies. If more than one X chromosome was active, the person would not develop normally.

5. It would have serious effects, because any recessive alleles carried on the maternal X chromosome would be expressed. If this were to happen, X-linked inheritance in men and women would be similar, and diseases such as hemophilia would be as high in women as in men.

6. A translocation had occurred that moved DNA sequences normally found on one chromosome to another.

7. This would allow the cell to concentrate the factors required for transcription and RNA processing, which would greatly increase the rate of interaction with their substrates and thus increase the efficiency of the processes.

8. Both mutations would block the ability of the repressor to bind to the DNA, thereby leading to the constitutive synthesis of the enzymes of the operon. Thus, the lactose-operon mutant would have the enzymes for lactose degradation even when the disaccharide was absent, whereas the histidine-operon mutant would have the enzymes for histidine biosynthesis even when the amino acid was present.

9. A mutation had occurred that changed the stop codon normally present at the end of the ß-galactosidase gene.

10. One way to test this would be to add an inhibitor of RNA synthesis (e.g., actinomycin D) at the same time as the hormone. If the production of myosin required new mRNA synthesis, the protein would not be synthesized.

11. No. It is just as likely that the transplanted nucleus could not adapt to the egg cytoplasm, which has a very different composition from that of the cytoplasm from which it had been removed. Negative results cannot provide definitive answers to an experimental question because one cannot be sure of the reason(s) for the results.

12. You could construct an affinity chromatography column using the DNA sequence as the immobilized ligand.

13. Enhancers are far enough away from the transcription initiation site to allow the DNA between them to form a loop, which allows the proteins bound at the enhancer to interact with the basal transcription machinery regardless of their orientation at the enhancer.

14. You could extract the RNA from the cell and see if it was capable of supporting protein synthesis in either an in vitro experiment or by injection into a cell, such as an amphibian oocyte, where it would be translated. If a cell is being inhibited translationally, it should contain an amount of mRNA disporportionate to its level of protein synthesis.

15. Many nuclear proteins move back and forth between the nucleus and cytoplasm which requires that they retain these localization signals. Even those nuclear proteins that remain in the nucleus during interphase must be reimported into the nucleus after mitosis following reformation of the nuclear envelope.

16. Because the methylated DNA has to recruit special methylated DNA-binding proteins, which in turn have to recruit corepressor complexes containing histone deacetylases, and the chromatin has to be converted to an inactive state. These events require a certain amount of time.

17. You could see which mRNAs are synthesized in a particular cell type under normal conditions and then determine which mRNAs are produced in a cell in which the transcription factor has been mutated, as in a knockout mouse lacking the factor entirely.

18. Donor nuclei obtained from adult epithelial cells are likely to have a considerably different pattern of DNA methylation than that of the DNA of a normal one-celled zygote. To support normal development, it is presumed that the methylation pattern of the donor nucleus must be modified so that it more closely resembles that of the zygote. The degree to which these modifications take place are likely to have a major impact on the success of the nuclear transplant







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